Conventionally, a high intensity discharge lamp (HID) such as a high-pressure mercury lamp, a metal halide lamp, a high-pressure sodium lamp or the like has mainly been used for an illumination device required to have a high output such as an illumination fixture installed at a high ceiling of a warehouse, a road illumination, or the like. With an increase in demand for energy saving, the illumination device is also required to save energy. Also in the HID, energy saving is proceeded by increasing efficiency through use of a metal halide lamp equipped with an arc tube made of translucent ceramics (ceramic metal halide lamp) or the like, but is not enough. The ceramic metal halide lamp degrades in intensity with time as with other HIDs and does not have a sufficient lifetime. The ceramic metal halide lamp thus has a disadvantage of high installation cost and maintenance cost.
As a long-lifetime and energy-saving illumination device, LED illumination is attracting attention. The LED illumination uses a light-emitting diode (LED) as a light-emitting source or an excitation source of phosphor. Therefore, the LED illumination has characteristics of less power consumption and a long lifetime of the order of several tens of thousands of hours to a hundred thousand hours. However, the LED illumination is generally widely used for a low-output illumination device but is regarded to be unsuitable for an illumination device required to have high output. In other words, when the LED illumination is made to have high output, its energy conversion efficiency degrades to increase its heat value, resulting in significantly shortened lifetime. When used as the illumination device for a high ceiling, the LED illumination is insufficient also in light distribution luminance.
Apart from the illumination device using an HID, LED or the like, a plasma illumination device having an electrodeless bulb is known. In the plasma illumination device, a light-emitting material filled in the electrodeless bulb is excited by a microwave for plasma emission. The plasma illumination device has, for example, a microwave generator, a microwave focuser to which the microwave generated in the microwave generator is guided, and an electrodeless bulb installed in the microwave focuser. The light-emitting material filled in the electrodeless bulb is excited by the microwave focused by the microwave focuser to the electrodeless bulb to thereby perform plasma emission. The electrodeless plasma illumination has a long lifetime because the light-emitting material filled in the bulb is activated with no physical contact, and is a point light source and therefore an illumination device suitable for light distribution design. However, the conventional plasma illumination device has a drawback that its luminous efficiency with respect to an input power is insufficient, and is therefore required to be improved in luminous efficiency and enhanced in total luminous flux based thereon.